I don't think whether he's on the ground or not is relevant. I think the ref must've determined that the block didn't take place from the front, or it took place with ball outside of the tacklebox and that Pierce was blocking him back toward the SU goal. I don't think either was true. Here's the relevant section of the NCAA rulebook.
Blocking Below the Waist ARTICLE 6. a.
Team A prior to a change of team possession:.
1. While the ball is inside the tackle box, the following players may legally block below the waist inside the tackle box until they leave the tackle box: linemen completely inside the tackle box, and stationary backs at least partially inside the tackle box and at least partially inside the frame of the body of the second lineman from the snapper. (A.R. 9-1-6-V)
2. While the ball is inside the tackle box, all other players (i.e., those not covered in paragraph 1), and all players after the ball has left the tackle box, are allowed to block below the waist only if the force of the initial contact is directed from the front. “Directed from the front” means within the clock-face region between “10 o’clock and 2 o’clock” forward of the player being blocked. Exceptions are given in paragraphs 3 and 4. (A.R. 9-1-6-I-II, IV, VII-VIII)
3. Players not covered in paragraph 1 (above) may not block below the waist toward the original position of the ball at the snap until the ball carrier is clearly beyond the neutral zone.
4. Once the ball has left the tackle box a player may not block below the waist toward his own end line. (A.R. 9-1-6-III)